The present invention relates to an antilock control device for efficiently operating the braking system of a motor vehicle even in case of a cadence brake.
An antilock brake control device is designed to detect the locking tendency of any wheel and reduce the hydraulic pressure in the brake wheel cylinder for the locking wheel to a lower level than the hydraulic pressure in a master cylinder controlled by the driver, thereby preventing the wheel from locking.
After the wheel has cleared out of the locking tendency, it is now necessary to increase the hydraulic pressure in the wheel cylinder because otherwise the braking force would be insufficient. But if the hydraulic pressure in the master cylinder is applied to the wheel cylinder too quickly, the wheel may soon fall into a locking state again. Thus, it is necessary to increase the hydraulic pressure in the wheel cylinders not sharply but gradually. For this purpose, it was proposed to provide the hydraulic circuit with two functions, i.e. the function of holding the pressure in the wheel cylinders and the function of bringing the wheel cylinders into direct communication with the master cylinder to increase the pressure in the wheel cylinders. With this arrangement, the hydraulic pressure in the wheel cylinders can be increased gradually while repeating the pressure increase and the pressure hold. For the same purpose, it was also proposed to provide a throttle valve or a flow control valve in the hydraulic circuit to selectively repeat a sharp pressure increase and a moderate pressure increase.
When the automobile is running on a rough surface, some of its wheels might be judged from the signals from the wheel speed sensors as if they are falling into a locking state in spite of the fact that the brake pedal is not operated, thus causing the antilock device to give a pressure reduction command. Even after the automobile has passed the rough surface and the wheels have been judged to be recovering from the looking state, the wheel cylinders are kept not in direct communication with the master cylinder as discussed above. If the brake pedal is trodden in such a situation, the hydraulic pressure generated in the master cylinder will not be transmitted to the wheel cylinders directly and smoothly, thus delaying the braking effect.
As one solution to this problem is to provide means for judging whether the brake pedal is in its operative or inoperative position by checking e.g. signals representative of the ON or OFF position of a stop lamp switch to keep the antilock device inoperative, i.e. keep the master cylinder in communication with the wheel cylinders as long as the brake pedal is in its inoperative position. This will allow the hydraulic pressure in the wheel cylinders to reach the pressure level in the master cylinder as soon as possible. But this solution has one drawback that the signals representative of the operative and inoperative positions of the brake pedal are not necessarily reliable. For example, the signal may remain OFF even if the brake pedal is actually put into its operative position, or it may remain ON even if the brake pedal is not trodden owing to the fusing of the switch.